This invention relates to a plasma facing component, namely, a plasma facing member to be exposed to plasma and, in particular, to a plasma facing member which is capable of withstanding plasma of a very high temperature in nuclear fusion reactors or the like.
In general, such a plasma facing member is located in nuclear fusion reactors or the like to guide or stop plasma of a very high temperature and is therefore inevitably exposed to the plasma.
Herein, it is to be noted that the plasma facing member is estimated by a loss which occurs on plasma radiation and which will be referred to as a plasma radiation loss. Specifically, the plasma facing member preferably has a low or reduced plasma radiation loss. Since the plasma radiation loss results from an impurity included in the plasma facing member, it is necessary to reduce an amount of the impurity in a material of the plasma facing member. The material of the plasma facing member will be called herein under a plasma facing material which includes Cu alloy, stainless steel, Nb alloy, V alloy, etc.
On the other hand, a low-Z material is known as a material which has a highly acceptable impurity concentration and has been therefore used as the plasma facing material. From this viewpoint, the low-Z material has been usually used as the plasma facing material.
Moreover, the plasma facing material must be strong against a thermal shock and must exhibit a high melting point, a low vaporization pressure, a high thermal conductivity, and a high mechanical strength. In addition, it is also required that the plasma facing material is rarely eroded for reducing the plasma radiation and is effective to recycle hydrogen which is used as fuel in nuclear fusion reactors or the like. This means that the plasma facing material preferably scarcely absorbs hydrogen.
Taking the above into account, graphite which has the atomic number of 6 and the low-Z material has been conventionally mainly used as the plasma facing material because the graphite has excellent thermal and mechanical stability and is stable even at a high temperature.
In the meanwhile, the graphite is disadvantageous in that a comparatively large amount of gas is discharged because the graphite is porous. In addition, erosion easily takes place in the plasma facing member of the graphite by ion sputtering due to radiation and by sublimation caused by plasma radiation. Especially, cracks appear in a large size of a tokamak device even when isotropic graphite is used.
In order to improve the above-mentioned disadvantages of the graphite, recent attention has been directed to tungsten as a candidate of the plasma facing material because tungsten has a high melting point and a low sputtering characteristic and is consequently small in gas discharge and in erosion. Such a plasma facing member of tungsten has been proposed in a paper which is contributed by T. Kuroda et al on International Atomic Energy Agency VIENNA 1991 and which is entitled "ITER PLASMA FACING COMPONENTS" in ITER DOCUMENTATION SERIES NO. 30.
As a rule, such a plasma facing member of tungsten is manufactured by refining tungsten by a powder-metallurgical method. It is noted that, on refining tungsten by the powder-metallurgical method, a gas component inevitably remains in the order of several tens of ppm in the refined tungsten together with alkali metal components of a low fusion point. The alkali components fall in the order from 0.2 ppm to several ppm within the refined tungsten. Moreover, an impurity, such as iron, which renders a grain boundary fragile is inescapably included in the tungsten.
On the other hand, a tungsten layer can be deposited by the use of a chemical vapor deposition (CVD) technique, as known in the art. However, consideration has not been made yet about manufacturing the plasma facing member by application of the CVD technique.